This invention relates to a demodulator for spread spectrum communication, comprising a matched filter including a surface acoustic wave device and a delay element.
A spread spectrum communication system which has less noise and superior secrecy has recently been watched with keen interest and development of transmitters and receivers for the spread spectrum communication have been launched. In the spread spectrum communication system, generally, a spread spectrum (SS) signal to be transmitted covers a very widely spread frequency band. More specifically, a carrier wave with narrow frequency band obtained by modulating information to be transmitted by a base band signal is further modulated by a series of codes with a predetermined high bit rate, thereby obtaining the spread spectrum signal with the frequency band. A pseudo random noise code series or a Gold code series is employed as the above-mentioned series of codes. Various systems have been proposed for the spread spectrum modulation, including a direct sequence method and a frequency hopping method.
The receiver is provided with a demodulator for demodulating the spread spectrum signals obtained by modulating the carrier waves with a series of the pseudo random noise codes in the direct sequence method, for example. The demodulator is arranged to derive the spread spectrum signal as an information bit signal when the spread spectrum signal received accords with a pattern of the pseudo random noise code series which is the same as that employed in modulating the information at the transmitter. In this case, since there is no interrelation between different pseudo random noise codes, the received signal is multiplied by the pseudo random noise code the same as that in the modulation, so that only the interrelating components of the transmitted signals can be delivered.
Since the spread spectrum signal modulated with the series of the predetermined codes has a very wide frequency band as compared with the frequencies used in conventional communication systems, the spread spectrum signal is less influenced by noise. Further, since the spread spectrum signal has a low power spectrum density and a better signal secrecy, it has advantage of being less likely to be intercepted. Additionally, since the spread spectrum modulation and demodulation are executed with the series of predetermined codes such as a series of pseudo random noise codes, frequency allocation is not necessitated for the purpose of preventing radio interference, as employed in the conventional communication system. Thus, the problem of shortage in the allocated frequency range with increase in the number of communication stations can be solved by spread spectrum communication.
Communication devices employing the above-described spread spectrum system have been exclusively produced for military use, because of its complicated arrangements and high costs. However, spread spectrum communication devices have recently been watched with a keen business interest, since a demodulator having a matched filter comprising a surface acoustic wave device has been found to be produced in a relatively simple arrangement. The usability of the spread spectrum communication has recently been reconsidered with an increased demand for consumer communication with weak radio waves in factories or offices. FIG. 2 schematically illustrates a direct sequence method type demodulator having a matched filter comprising a surface acoustic wave device.
Referring to FIG. 2, a matched filter 1 composed of a surface acoustic wave device has an input electrode 3 and an output electrode 4 formed on a piezoelectric substrate 2 with a predetermined distance between them. The input electrode 3 has an interdigital pattern corresponding to the pseudo random noise codes employed for the spread spectrum modulation. For example, when the series of pseudo random noise codes is a series of n-bit pulses, the interdigital pattern of the input electrode 3 is formed so as to correspond to the n-bit pattern. Consequently, when the spread spectrum signal is supplied to the input electrode 3, the signal is transferred as surface acoustic waves through the surface portion 2a of the piezoelectric substrate 2. The magnitude of the surface acoustic waves is increased when the coded pattern of the spread spectrum signal corresponds to the pattern of the input electrode 3. More specifically, the spread spectrum signal is picked up at the output terminal 4 as an information signal every time the spread spectrum signal is matched with the pattern of the input electrode 3. Thus, the information signals are periodically delivered. An amplifier 5 has an input electrode connected to the output electrode 4 of the matched filter 1. The periodical information signals appearing at the output electrode 4 are amplified by the amplifier 5 to be delivered. A delay path 6 comprising a surface acoustic wave device has interdigital input and output electrodes 8 and 9 formed on an piezoelectric substrate with a predetermined distance between them. When an electrical signal is supplied to the input electrode 8, it is transferred to the output electrode 9 through a surface portion 7a of the piezoelectric substrate 7 as the surface acoustic wave having the propagation velocity slower than that of the electrical signal. In this case the delay path 6 is so set that a delay signal delayed by one cycle relative to each information signal periodically supplied from the matched filter 1 is obtained. A product of the outputs of the matched filter 1 and the delay path 6 is obtained by a product circuit 10, thereby demodulating the spread spectrum signal into the carrier wave to be delivered.
In accordance with the above-described arrangement, when the spread spectrum signal modulated in the direct sequence system, the spread spectrum signal is transmitted, as a large surface acoustic wave, to the output electrode 4 through the piezoelectric substrate surface 2a every time the pattern of the series of codes corresponds to the pattern of the interdigital input electrode 3. The output from the output electrode 4 is supplied to the amplifier 5 as the information signal and also to the product circuit 10. On the other hand, the information signal supplied to the delay path 6 from the matched filter 1 through the amplifier 5 is delivered to the product circuit 10 as a delay signal with a delay of one cycle. Consequently, the information signal from the matched filter 1 is detected in the product circuit 10 in synchronism with the delay signal from the delay path 6 such that the information signal is demodulated to the usual carrier wave output.
Generally, the surface acoustic wave device has a disadvantage that the insertion loss is large or the conversion efficiency of the electrical signal to the surface acoustic wave signal is low. Accordingly, the amplifier 5 is necessitated when the information signal is input from the matched filter 1 to the delay path 6. Such a large insertion loss also reduces the S/N ratio. Further, installation of the matched filter 1 and the delay path 6 each as the surface acoustic wave device and the amplifier 5 prevents the demodulator device from being rendered small-sized.